Focused ion beam systems can be used in forming, shaping or altering microscopic structures in semiconductor devices or other solid materials. The focused ion beam is directed to a small point on a semiconductor device and then scanned, raster fashion, over a particular area where material is to be removed or deposited. As an ion impinges upon the semiconductor device surface, its momentum is transferred and can result in the removal of one or more surface atoms according to a process called sputtering. By selecting a raster pattern of a given overall shape, for example a horizontal raster pattern, a correspondingly shaped area of surface material can be removed or conductive material can be deposited if a particular metal containing compound is adhered to the semiconductor device. Insulating material may also be deposited upon a specific area of a substrate by directing an ion beam toward a substrate simultaneously with the introduction of a gaseous compound. The beam irradiated upon the substrate in the presence of the gaseous compound forms an insulating film.
Any of the various layers of a semiconductor device may be operated upon with a FIB system, for example, by removing material down to the layer of interest, and selectively adding conductive or insulative material or removing portions of the same so as to accomplish the desired modification of the semiconductor device. However, subsurface features of objects are often not visible in focused ion beam images. Heretofore, in semiconductor device design and manufacture, for example, it has been possible to use a focused ion beam system to image the surface of the semiconductor device. Portions of subsurface layers of the device could be located via specific landmarks on the device being observed by using the topography of the surface to direct the positioning of the ion beam in relation to subsurface features. Referring now to FIG. 1, which is a cross section view of a portion of a device constructed using previous manufacturing styles, a pair of conductors 12 are deposited on a substrate 10 and are covered by an insulation layer 14. It may be observed at location 16, for example, the insulation layer has a series of hills and valleys formed therein as a result of deformations from passing over the top of the conductors 12. Accordingly, an adequate FIB image can be obtained since the hills and valleys provide sufficient detectable topography to enable the locating of subsurface features.
However, recent fabrication methods involve planarization, as illustrated in FIG. 2, wherein the upper layer 14 has been planarized, by polishing or the like, to create a smooth surface topography so as to provide a level surface for subsequent layer deposition. Without planarization, additional layers which are added on top of existing layers develop increasingly steeper hills and valleys which can result in undesirable performance characteristics for the electronic device being manufactured. However, the planarization of the various layers removes any variation in topography of subsequently deposited layers resulting in difficulty in locating subsurface features through observation with a focused ion beam system.
Accordingly, when planarization is employed to provide a smooth surface on which to deposit further layers, in accordance with the prior art, one method employed to overcome the inability to observe below surface features of planarized devices with an ion beam system has been to employ a high precision X-Y stage upon which the sample being observed is mounted. Then, precise movements are employed from known starting or reference points in order to position the sample below the ion beam so as to have the beam strike the sample at the desired position. However, high precision stages are expensive and add significantly to the cost of a system.